Controversy exists regarding the effect of gender on differences in surgical infectious disease outcomes. Previously, our laboratory reported that the gender-based difference in mortality was greatest among critically ill patients who developed nosocomial pneumonia. Therefore, the proposed prospective, observational cohort study will concentrate on these hospital-acquired infections, stratifying patients not only by gender, but also by estrogen-status. Based on evidence that women who develop nosocomial pneumonia are at higher risk for death, especially if they are elderly, we hypothesize that estrogen-rich pre-menopausal women will be least likely to develop nosocomial pneumonia, while post-menopausal women will have the highest mortality. Thus, we hope to confirm our earlier findings of the incidence and mortality related to nosocomial pneumonia, and to further evaluate three areas of potential gender differences to account for the disparate outcomes: diagnosis, treatment and immune response. We believe that these areas are not only influenced by sex-based biologic and physiologic features, but also social factors. In the future, these data may lead to methods that facilitate either prevention, or earlier diagnosis and more efficacious treatment of nosocomial pneumonia, a disease which accounts for a substantial proportion of the lethal infections in surgical patients.